 Drawing
is a metalworking process which
uses tensile forces to stretch metal.
 It is broken up into two types: sheet metal drawing and
wire, bar, and tube drawing.
 For wire, bar, and tube drawing the starting stock is
drawn through a die to reduce its diameter and
increase its length.
 Drawing is usually done at room temperature, thus
classified a cold working process, however it may be
performed at elevated temperatures to hot work large
wires, rods or hollow sections in order to reduce forces
 Drawing operations involve pulling metal through a
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die by means of a tensile force applied to the exit side
of the die.
The plastic flow is caused by compression force,
arising from the reaction of the metal with the die.
Starting materials: hot rolled stock (ferrous) and
extruded (nonferrous).
Material should have high ductility and good tensile
strength.
Bar, wire and tube drawing are usually carried out at
room temperature, except for large deformation,
which leads to considerable rise in temperature during
drawing.
The metal usually has a circular symmetry (but not
always, depending on requirements).
 Wire
drawing
involves
reducing the diameter of a
rod or wire by passing
through a series of drawing
dies or plates.
 The subsequent drawing die
must have smaller bore
diameter than the previous
drawing die
 Tube drawing involves reducing the cross section and
wall thickness through a draw die
 The cross section can be circular, square hexagonal or
in any shapes
 Shape of the bell causes hydrostatic
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pressure to increase and promotes
the flow of lubricant into the die.
The approach angle – where the
actual reduction in diameter
occurs, giving the half die angle α.
The bearing region produces a
frictional drag on the wire and also
remove surface damage due to die
wear,
without
changing
dimensions.
The back relief allows the metal to
expand slightly as the wire leaves
the die and also minimises abrasion
if the drawing stops or the die is out
of alignment.
The die nib made from cemented
carbide or diamond is encased for
protection in a thick steel casing
 Most drawing dies are cemented
carbide or industrial diamond (for
fine wires).
 Cemented carbides are the most
widely used for drawing dies due
to
their
superior
strength,
toughness, and wear resistance.
 Cemented carbide is composed of
carbides of Ti, W, Ni, Mo, Ta, Hf.
 Polycrystalline Diamond (PCD)
used for wire drawing dies – for
fine wires. Longer die life, high
resistance to wear, cracking or
bearing.
 Bull block drawing allows
the generation of long
lengths
 Area reduction per drawing
pass is rarely greater than
30-35%.
 More economical design.
 Use a single electrical motor to drive a series of
stepped cones.
 The diameter of each cone is designed to produce a
peripheral speed equivalent to a certain size reduction.
 From the uniform-deformation energy method, a draw
stress is given by
Eq. 1
 Consider the problem of strip drawing of a wide sheet
A wide strip is being drawn
through a frictionless die with
a total included angle of 2α
Plane strain condition is applied
(no strain in the width direction.)
The equilibrium of forces in the x
direction is made up of two components
1. Due to the change in longitudinal stress with x increasing positively to the
left.
2. Due to the die pressure at the two interfaces.
Taking equilibrium of force in the x direction and neglecting dσx dh
Eq.2
We shall now consider the problem of strip drawing where a
Coulomb friction coefficient exists between the strip and
the die.
The equilibrium now includes 2µpdx
Taking equilibrium of forces in the x direction eq.2 becomes
Since h = 2x tan α, and dh = 2dx tanα, then 2dx = dh/tan α
We now have
Eq. 3
Since the yield condition for plane strain is σx + p = σ’0 and
B = µ cot α, the differential equation for strip drawing is
Eq. 4
If B and σ’0 are both constant, Eq.4 can be integrated
directly to give the draw stress σxa
For wire drawing conducted with conical dies,
Eq. 5
The surface area of contact between
the wire and the die is given by
Eq. 6
 p is the mean normal pressure on this area.
 Pd is the draw force
Balancing the horizontal components of the frictional
force and the normal pressure.
Eq. 7
In the asence of friction, B = 0 and
Eq. 8
The draw stress with friction is given by
Eq. 9
If redundant work is included in Eq. 9, the expression
becomes
Eq. 10
Where Ф is a factor for the influence of redundant work,
which can be defined as
Eq. 11
Where ε* = the enhanced strain corresponding to the yield
stress of the metal, which has been homogeneously
deformed to a strain ε
 Following
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the hot forming
process, tubes are cold drawn
using dies, plugs or mandrels to
the
required
shape,
size,
tolerances
and
mechanical
strength.
provides good surface finishes.
increase mechanical properties
by strain hardening.
can produce tubes with thinner
walls or smaller diameters than
can be obtained from other hot
forming methods.
can produce more irregular
shapes.
There are three basic types of tube-drawing processes
1. Sinking
2. Plug drawing
- Fixed plug
- Floating plug
3. Mandrel drawing
 The tube, while passing through the die, shrinks in outer
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radius from the original radius R0 to a final radius R0f
No internal tooling (internal wall is not supported), the wall
then thicken slightly.
Uneven internal surface.
The final thickness of the tube depends on original diameter
of the tube, the die diameter and friction between tube and
die.
Lower limiting deformation.
 Use cylindrical / conical plug to control size/shape of inside
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diameter.
Use higher drawing loads than floating plug drawing.
Greater dimensional accuracy than tube sinking.
Increased friction from the plug limit the reduction in area
(> 30%).
can draw and coil long lengths of tubing.
 A tapered plug is placed inside the tube.
 As the tube is drawn the plug and the die act together
to reduce both the outside/inside diameters of the
tube.
 Improved reduction in area than tube sinking (~ 45%).
 Lower drawing load than fixed plug drawing.
 Long lengths of tubing is possible.
 Tool design and lubrication can be very critical.
 Draw force is transmitted to the metal by
the pull on the exit section and by the
friction forces acting along the tube –
mandrel interface.
 minimised friction.
 The mandrel also imparts a smooth inside
finish surface of the tube.
 mandrel removal disturbs
dimensional tolerance.
 Longitudinal scratches (scored die, poor lubrication ,
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or abrasive particles)
Slivers (swarf drawn into the surface).
Long fissures (originating in ingot).
Internal cracks (pre-existing defects in starting
material or ruptures in the centre due to overdrawing).
Corrosion induced cracking due to internal residual
stresses.
centre burst or chevron cracking (cupping).
 Yield Strength of the material
 Cone angle
 Reduction ratio
 Coefficient of friction
 Force